A method of encoding an audio signal for transmission and/or for storage
includes changing a digitized audio signal based on a selected signal
changing criteria. The changed digitized audio signal is encoded, based at
least on an available data rate for transmitting and/or storing the audio
signal, for reducing an amount of data associated with the changed,
digitized audio signal. The reduced-data audio signal is then evaluated
for determining whether a code overload condition exists. If a code
overload condition exists, another signal changing criteria is selected. A
time behavior and/or a spectral distribution of the digitized audio signal
is recursively changed based on the another signal changing criteria
before the amount of data associated with the digitized audio signal is
reduced so that deterioration of reduced-data audio signal is not
perceptible when the coded signal is decoded. The recursively changed
digitized audio signal is encoded for reducing an amount of data
associated with the recursively changed digitized audio signal based at
least on the available data rate. Lastly, the recursively changed encoded
reduced-data digitized audio signal is transmitted and/or stored.

1. A method of encoding an audio signal for transmission and/or for
storage, the method comprising the steps of:

changing a digitized audio signal based on a selected signal changing
criteria;

encoding the changed digitized audio signal for reducing an amount of data
associated with the changed digitized audio signal based at least on an
available data rate for transmitting and/or storing the audio signal;

recursively changing a time behavior and/or a spectral distribution of the
digitized audio signal based on the another signal changing criteria
before the amount of data associated with the digitized audio signal is
reduced so that deterioration of reduced-data audio signal is not
perceptible when the recursively changed digitized audio signal is decoded
after transmission and/or storage;

encoding the recursively changed digitized audio signal for reducing an
amount of data associated with the recursively changed, digitized audio
signal based at least on the available data rate; and

2. The method according to claim 1, wherein the step of selecting another
signal changing criteria comprises the steps of:

expanding the reduced-data digitized audio signal;

generating a difference signal between the digitized audio signal and the
expanded digitized audio signal; and

evaluating the digitized audio signal, the difference signal and the
expanded digitized audio signal for selecting the another signal changing
criteria.

3. The method according to claim 2, further comprising the steps of:

evaluating the digitized audio signal, the difference signal and the
expanded digitized audio signal for selecting an encoding strategy for
encoding the recursively changed digitized audio signal; and

selecting the encoding strategy based on a table of interference effect
patterns.

4. The method according to claim 3, further comprising the step of delaying
the digitized audio signal for a predetermined period of time before
recursively changing the digitized audio signal.

5. The method according to claim 2, wherein the another signal changing
criteria includes at least one of a frequency response change, a temporal
change, and a signal repetition in place of an actual signal.

6. The method according to claim 2, wherein the another signal changing
criteria includes a simplification of a spatial imaging of multichannel
audio signals when the audio signal comprises a plurality of audio
signals.

7. The method according to claim 1, wherein when the audio signal is a
stereophonic signal or two or more audio signals, at least one of the
steps of changing the digitized audio signal and recursively changing the
digitized audio signal is based on mutual level relationships of the audio
signals.

8. The method according to claim 1, wherein when the available bit rate is
a low bit rate, at least one of the steps of changing the digitized audio
signal and recursively changing the digitized audio signal causes a speech
intelligibility of the audio signal to at least be maintained or to be
improved.

9. The method according to claim 1, wherein at least one of the steps of
encoding comprises the step of assigning code bits based on a previous
change of the digitized audio signal.

10. A method according to claim 1, further comprising the step of repeating
the steps of the method after a predetermined period of time.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of transmitting and/or storing digitized,
data-reduced audio signals. Such a method is known, for instance, from the
European patent specification 290 581.

2. Description of the Related Art

In a bit rate-reduced encoding of audio signals that are already available
in a digitized form, e.g., having a 48 kHz scanning frequency/16 bit
resolution, utilization of psychoacoustic phenomena in the perception of
audio signals is known from EP 290 581 in a way that the original bit rate
is considerably reduced. Such methods carrying out transcoding from a
higher to a lower bit rate are commonly known as source coding. They
particularly utilize masking effects of the ear in the frequency and time
domain. It is the object of these methods to not change, if possible, the
audible quality which the audio signal has before transcoding (source
coding) by reducing the bit rate (optimum transcoding).

In many applications, there is a limited data capacity available for
transmission or storage. If this bit rate, which is made available, is
insufficient, a further data reduction according to the principles of
optimum transcoding leads to sometimes considerable interference,
especially in the form of considerable sound distortions. This same
interference, of course, also occurs if the source coding is not
two-staged, but takes place in one step.

Furthermore, in a method of transmitting and/or storing digitized,
data-reduced audio signals it is known (DE 36 29 434 Al) to let further
encoding steps follow if, after encoding, a predetermined bit rate is
exceeded (code overload), until the actual bit rate no longer exceeds the
designated bit rate. That is, if code overload exists after a first
encoding sep, further encoding steps can be employed. However, this
approach does not always avoid a code overload with certainty.

SUMMARY OF THE INVENTION

In contrast, it is the object of the present invention to minimize the
interference or distortion effect during a further data reduction of an
already data-reduced audio signal or during a first data reduction of an
audio signal, while accepting code violations.

This object is provided by the present invention so that in the event of a
code overload, before the audio signal is encoded, the time behavior
and/or the spectral distribution of the audio signal is changed according
to a selected adaptation pattern without a considerable signal
deterioration being subjectively perceivable after decoding the encoded
audio signal.

Advantageous modifications of the solutions according to the invention are
described as follows.

BRIEF DESCRIPTION OF THE DRAWING

By way of an embodiment, the invention is explained in greater detail in
the only drawing. It shows:

FIG. 1 is a block diagram of an encoder for the implementation of the
method according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the encoder shown in FIG. 1, the arriving analog audio signal 11 is fed
to an analog/digital conversion stage 10. The analog signal 11 has various
signal features, such as a spectral distribution and a time behavior (or
behavior during the course of time). The digitized audio signal 12 is fed
to a stage 40, to be explained later, for signal analysis, to a signal
change stage 20, as well as to delay elements 15 and 70. In the signal
change stage 20, the digitized audio signal is processed as a function of
a change recommendation 41 generated by the signal analysis stage 40. The
change recommendation 41 may also be to not change anything. The processed
and possibly changed audio signal 21 at the output of stage 20 is reduced
in a subsequent data reduction stage 30 with regard to the data rate
available for transmission or storage. This data reduction is carried out
as a function of an encoding recommendation 42 which is made available by
the signal analysis stage 40. The encoded audio signal 31 at the output of
stage 30 is fed to a data expansion stage 50 for use in a recursive
control process, which will be described in greater detail below, in which
the data-reduced audio signal is expanded so that, at the output of this
stage 50, a digitized audio signal 51 is available, in a linearly coded 16
bit pulse code modulation. This re-expanded digital audio signal 51 is fed
to the signal analysis stage 40. In addition, there are applied to the
input of stage 40 the digitized audio signal 12 to be encoded as well as
an information signal 32, supplied by the data reduction stage 30,
concerning the data rate required for the encoding of the audio signal 12,
with the data rate providing information on the existing code violations
during the data reduction of the audio signal 12. Furthermore, stage 40 is
fed a differential signal 13 which results from the generation of the
difference between the re-expanded audio signal 51 and the audio signal 12
to be encoded. In order for the differential generation to relate to
temporally synchronous audio signals, the audio signal 12 to be encoded is
delayed by the amount .DELTA.t.sub.1 in the delay stage 70 and is also fed
to stage 40. Based on the information signal 32, the signal analysis stage
40 determines whether a code overload is present. A code overload is
defined as a state in which the available capacity of a transmission or
storage medium is not sufficient for the optimum transcoding of the audio
signal. If such a code overload is present, the recursive control process
is employed, with the input signals 12, 13 and 51 applied to the signal
analysis stage 40 being examined with regard to possible signal changes as
well as to encoding strategies. Possible signal change measures, or
measures for changing the signal changes or criterion, that are features
of the audio signal (unless the signal change measure indicates no
change), communicated to the signal change stage 20 in the form of a
change recommendation 41, limitation or a time behavior change such as a
level change of the audio signal to be encoded, which lead to a reduction
of the data rate required in the data reduction stage 30 and, at the same
time, to a minimum interference effect of the re-expanded audio signal 51.
Apart from the bandwidth limitation and level change mentioned, further
typical signal change measures include other spectral distribution changes
such as frequency response changes (pre-emphasis), and other time behavior
changes such as steeper or lesser signal amplitude changes, or signal
repetitions in place of the actual signal, particularly for periodic or
quasi-periodic signal characteristics of the audio signal. Simplification
of the spatial imaging or distribution of multichannel audio signals, such
as providing mono instead of stereo for the entire audio frequency range
or for a part of the frequency range, may also be used as a signal change
measure. A signal change measure selected by the signal analysis stage 40
is communicated to the signal change stage 20 in the form of a change
recommendation 41. The signal analysis stage 40 also generates an encoding
recommendation 42 which is communicated to the data reduction stage. The
encoding recommendation 42 is selected from a table of distortion or
interference effect patterns in the signal analysis stage 40 by using the
change recommendation 41 generated there. The encoding recommendation 42
is used in the data reduction stage 30 for a transcoding of the audio
signal to be encoded at a reduced bit rate so that the interference or
distortion produced will be as small as possible. The recursive control
process required for the signal analysis in stage 40 can also be carried
out several times, if necessary, until a satisfactory result is achieved
by the change and encoding recommendations 41, 42. As will be apparent
from the foregoing, if a code overload condition is detected then at least
one signal feature of the audio signal is recursively changed. For the
final signal change and data reduction, the final change and encoding
recommendations 43 or 44 are fed to a further signal change stage 22 or a
further data reduction stage 33, which function in the same manner as the
stages 20 and 30. Since a certain amount of time is necessary for the
recursive control process, the audio signal 12 must be time-delayed by the
amount .DELTA.t.sub.2 before the final suboptimum transcoding by means of
stage 15. The data-reduced audio signal 34 resulting from the data
reduction stage 32 can be transmitted, e.g., to a channel encoder, which
is not shown, for storage on a sound carrier or for the processing of a
broadcast signal. During the encoding of the audio signal, code bits are
assigned to the data-reduced encoded signal based on the recommended
changes made to the digitized audio signal.

When very a low bit rate of the data-reduced audio signal is available, the
purposeful change of the digital signal prior to encoding is effected so
that the speech intelligibility of the audio signal is maintained or is
improved, if possible. During a stereophonic encoding, or when two or more
audio signals are encoded together or as a function of one another, the
purposeful change of the various audio signals occurs prior to the data
reduction/encoding of the signals, with the change being based on mutual
level relationships of the audio signals. The selection of the particular
encoding scheme and/or the particular purposeful change of the audio
signal prior to encoding are/is respectively determined anew after short
periodic time intervals.

The method according to the invention can also function without recursive
control process if an internal examination of the audio signal 51 which
has been data-reduced and re-expanded by the change and encoding
recommendations 41 or 42 is dispensed with. The advantage related herewith
of reduced physical intricacy (complexity), however, is gained at the
expense of the achievable optimum of the suboptimum transcoding. If the
recursive control process is dispensed with, the input signals 13 and 51
at the signal analysis stage 40, as well as the time-delayed audio signal
12, are deleted. The already encoded signal 31 represents the final
encoded audio signal so that also the stages 15, 22 and 33 are not
required. A further simplification of the method according to the
invention is achieved by, in addition, dispensing with the continuously
conducted signal analysis, by, e.g., setting the signal change stage 20 to
a constant change and the data reduction stage 30 to a constant encoding
recommendation.

It is understood that the signal processing that may have to be made in the
signal change stage 20 can also be carried out prior to the analog/digital
conversion of the analog audio signal 11. For this purpose, the stage 20
may be re-located so that it is connected in series before the
analog/digital converter.